Cancer and its treatment are associated with considerable distress, impaired quality of life (QOL), poor mental health, and reduced physical function. This is particularly true for women with breast cancer who receive multimodal treatment over an extended period of time. Many women undergo surgery and chemotherapy, which are often followed by radiotherapy. It is important to develop programs that can help ameliorate the treatment-related morbidity that accumulates over time for women with breast cancer. Furthermore, given the stressful time demands posed by radiotherapy treatment (patients typically receive treatment 5-days per week for 6 weeks) it is crucial that programs be easily incorporated into the treatment schedule. Research suggests that stress-reduction programs tailored to the cancer setting may help patients cope with the acute effects of treatment and improve QOL after treatment. Yoga, an ancient Indian science, incorporates stress-reduction techniques including regulated breathing, visual imagery, meditation, and various gentle stretching postures. Yoga may be particularly useful for women with breast cancer after surgery and while undergoing radiation treatment because of the stress management and relaxation techniques and the gentle stretching that should facilitate recovery. Preliminary studies found yoga is useful for women with breast cancer undergoing radiotherapy. Integrating a yoga program into the treatment regimen of women undergoing radiotherapy for breast cancer makes participation in the program easy and compliance has been high. We will determine the efficacy of implementing a yoga program for patients with breast cancer as an adjuvant to their radiotherapy. Patients with breast cancer who are undergoing radiotherapy will be randomly assigned to one of three groups: a yoga group, a stretching/relaxation group, or a waitlist control group. Participants in the two intervention groups will attend sessions 3 days/week throughout their 6-week radiotherapy schedule. Measures will be obtained prior to randomization, midway through radiotherapy, during the last week of radiotherapy, and 1 and 3, 6 and 12 months after the end of radiotherapy. We will examine indices of QOL, fatigue, sleep disturbances, mental health, cost- effectiveness analysis, work and/or home productivity, and cortisol rhythmicity. We hypothesize that the yoga program will help facilitate recovery and alleviate the physiological and psychological side effects experienced by patients who are currently receiving radiotherapy.